This invention relates generally to stereo sound signals and specifically to a simple stereo volume control and method for use in a television signal decoder.
Recently, stereo sound and second audio programming (SAP) have been authorized by the FCC for television signal transmissions. The Broadcast Television Systems Committee (BTSC)-approved multi-channel sound signal comprises preemphasized left (L)+right (R) stereo signal components, L-R stereo signal components, AM modulated on a 2.0 f suppressed subcarrier, a 1.0 f pilot signal component for regenerating the suppressed subcarrier and a 5.0 f SAP FM subcarrier component. The frequency f corresponds to the horizontal line frequency of a television signal, namely 15.734 KHz. The L+R information is thus below 15.734 whereas the L-R subcarrier is centered at 31.468 KHz and the SAP subcarrier is at 78.67 KHz. During encoding the L-R signal is compressed to enhance its signal to noise characteristics and thereby improve received sound quality. The transmitted audio signal is thus more than a simple combination of sum and difference signals but has the sum and difference components weighted in a non-linear manner. Thus providing a volume control that maintains proper stereo separation and proper left and right signal balance is quite difficult.
In a stereo television receiver, the BTSC signal is partially decoded, forming the deemphasized [L+R] signal and the still compressed [L-R] signal. The [L-R] signal is then expanded and matrixed with the [L+R] signal to form L and R signal components which may then be individually and simultaneously controlled to adjust their respective loudness levels. The SAP signal, if any, is separately detected and reproduced.
Many viewers receive cable television signals by means of a converter/decoder provided by the cable systems operator. The converter/decoder is interposed between the cable-connected television signals and the subscriber's television receiver. The cable operator may also provide a remote control option with the converter/decoder to enable subscribers to control channel selection, volume, etc. from a remote point. There are also stereo adapters for enabling subscribers having "monaural" television receivers to enjoy television transmissions with stereo sound. As will be seen, the remotely controllable functions on appropriately equipped television receivers are rarely fully used because of the presence of the converter/decoder.
Cable signals are supplied at selected television carrier frequencies, demodulated to baseband frequency, remodulated to a frequency corresponding to VHF channel 3/4 and coupled to the viewer's television receiver antenna input terminals. Thus the television receiver is always tuned to either channel 3 or channel 4 and a remote channel change feature is of little practical value. A similar situation generally obtains with volume control. For monaural audio, the decoder may decode the television audio signal to baseband frequency and enable direct volume control before FM modulating it to a 4.5 MHz carrier, which then becomes part of the composite video signal and is remodulated to channel 3/4. With a BTSC stereo signal, however, a major difficulty is encountered in attempting to control volume in the decoder. Because of the compressed nature of the signal, the BTSC signal must be decoded, expanded and matrixed to obtain the correct L and R signal components. The L and R signal amplitudes may then be varied for volume control and reencoded into the BTSC stereo format, FM modulated onto a 4.5 MHz carrier and subsequently remodulated, along with the other parts of the television signal, to channel 3/4. A stereo television receiver is equipped to process the BTSC stereo signal and reproduce the gain-controlled L and R signals. Similarly, if a separate stereo adapter is used with the decoder, the adapter will include the requisite BTSC decoding circuitry. It is readily apparent that it is a significant complication and expense for a cable operator to provide the necessary BTSC decoding and encoding audio circuitry in the decoder to enable volume control by means of the decoder remote control unit.
Accordingly, there is a need in the art for a simple BTSC stereo volume control for use in a television decoder.